In recent years, prevalence of portable terminals such as a portable phone, a notebook type personal computer, and a pad type information terminal apparatus is considerable. A non aqueous electrolyte battery is used as a secondary battery that is used as a power source for these portable terminals. Since a more comfortable portability is demanded in the portable terminals, scale reduction, thickness reduction, weight reduction, and performance enhancement of the portable terminals are rapidly developing, and the portable terminals are coming to be used in various fields. This trend is currently still continuing, and further scale reduction, thickness reduction, weight reduction, and performance enhancement of batteries used in the portable terminals are also demanded.
A non aqueous electrolyte battery has a structure such that a positive electrode and a negative electrode are disposed via a separator and housed in a container together with an electrolytic solution obtained by dissolving a lithium salt such as LiPF6, LiTFSI (lithium (bistrifluoromethylsulfonylimide)), or LiFSI (lithium (bisfluorosulfonylimide)) into an organic liquid such as ethylene carbonate.
Typically, the positive electrode and the negative electrode are formed in such a manner that a slurry for electrodes, which is obtained by dissolving or dispersing a binder and a thickening agent into water and mixing the resulting material with an active material, an electroconductive auxiliary agent (electroconductivity imparting agent) that is added in accordance with the needs, and the like (the slurry for electrodes may hereafter simply referred to as a slurry), is applied onto a current collector, followed by drying water to bind as a mixed layer. More specifically, for example, the positive electrode is formed in such a manner that LiCoO2 or the like serving as an active material, carbon black serving as an electroconductive auxiliary agent that is added in accordance with the needs, and the like are bound with each other onto a current collector such as an aluminum foil with use of a binder for secondary battery electrodes. On the other hand, the negative electrode is formed in such a manner that a carbonaceous material serving as an active material capable of intercalating and deintercalating lithium ions and an electroconductive auxiliary agent that is similar to that of the positive electrode and added in accordance with the needs, and the like are hound with each other onto a current collector such as copper with use of a binder for secondary battery electrodes.
Hereto, polyvinylidene fluoride (PVdF) is typically used as a binder for positive electrodes (for example, Patent Literature 1). However, PVdF necessitates N-methyl-2-pyrrolidone (NMP) serving as an organic solvent, thereby raising a problem in terms of burden on the environment. In addition, PVdF swells by dint of the electrolytic solution when present under a high-temperature environment of 50° C. or higher, whereby the binding force weakens, and also the electric resistance increases, so that PVdF is deficient in terms of high-temperature durability.
On the other hand, in the negative electrodes, use of an aqueous binder is already foregoing for consumer purposes (personal computers, portable phones, and the like), and examples thereof include diene-based rubbers such as styrene-butadiene rubber and acrylic series such as polyacrylic acid (for example, Patent Literatures 2 and 3). Examples of the thickening agent include methyl cellulose, ethyl cellulose, hydroxyethyl cellulose, hydroxypropoxycellulose, carboxymethyl cellulose-sodium salt (CMC-Na), and sodium polyacrylate. Among these, CMC-Na is often used (for example, Patent Literature 4).
The reasons why it is difficult to use an aqueous binder in the positive electrodes, unlike in the negative electrodes, are as follows:
(1) the aqueous binder undergoes oxidation decomposition when the battery is charged;
(2) it is difficult to disperse the slurry uniformly;
(3) when it is attempted to increase the positive electrode capacity by thick coating, cracks are generated in the electrode due to cohesion stress caused by drying. As a result, a sufficient electric conduction path is not ensured in the electrode, so that the high-rate discharge characteristics decrease as the battery characteristics;
(4) by contact and reaction of the positive electrode active material with water, lithium serving as a positive electrode active material elutes, and cracks are generated in the electrode, leading to decrease in the positive electrode capacity and the cycle characteristics;
and the like.
The present invention has been made in view of the aforementioned circumstances, and an object thereof is to provide a slurry composition for non aqueous electrolyte batteries that is electrically and thermally stable even when an aqueous binder is used, and that can achieve a high discharge capacity retention ratio without decreasing the positive electrode active material capacity.